The ability to determine oxygen saturation of hemoglobin in different parts of a person or animal's body is useful for a variety of known medical reasons. One method of determining oxygen saturation of hemoglobin is through transcutaneous near infrared spectroscopy (NIRS). Various types of equipment that use NIRS to measure oxygen saturation of hemoglobin in arteries and tissue are known to those of skill in the art and are commercially-available. For example, Somanetics Corporation of Troy, Mich. commercially offers for sale its INVOS System, which uses NIRS to estimate regional hemoglobin oxygen saturation in tissue, including in the brain, muscles and organs, such as the kidney or gut. U.S. Pat. Nos. 6,615,065 and 5,902,235 assigned to Somanetics Corporation, further illustrate devices for measuring oxygen saturation of hemoglobin in tissue. Devices that measure oxygen saturation of hemoglobin, such as Somanetics' INVOS system and the devices described in the above-referenced patents owned by Somanetics are generally referred to as oximeters.
Oximeters that measure oxygen saturation in the arteries are referred to as pulse oximeters while tissue oximeters such as the Somanetics INVOS System are useful for measuring venous weighted oxygen saturation of hemoglobin in regional tissue. It is widely accepted in the medical arts that the venous oxygen saturation reflects the balance between oxygen delivery to the tissue and tissue demand for oxygen. There is a long-standing medical need to be able to determine whether a change in the oxygen saturation of hemoglobin in various regional tissue areas is the result of systemic oxygen delivery conditions, including but not limited to hypoxemic hypoxia, hypovolemic hypoxia, anemia and the like or local site-specific oxygen delivery conditions, such as restricted blood flow to a particular organ. For example, differentiating between systemic oxygen delivery conditions and local site-specific oxygen delivery conditions may be important in the management of pediatric patients who have congenital heart disease; in identifying patients who are at risk for developing necrotizing enterocolitis (NEC); and in assisting surgeons to adjust procedures to prevent organ damage from ischemia.
Those skilled in the art will be aware that changes in the delivery of oxygen can be due to changes in the blood and hemoglobin oxygen transport function as well as to various changes in the physiologic systems that deliver blood to the tissue and hence changes in tissue oxygen saturation will reflect changes in those functions and systems and may be used to monitor functions and systems. Those persons skilled in the art will recognize many other benefits of being able to determine whether a change in oxygen delivery to the tissues is due to systemic or local causes
The inventive methods and systems described hereinafter provide a method and system for differentiating between global or systemic hypoxia on the one hand and local or site-specific tissue ischemia on the other hand.